As it is known, gas dehumidifiers, typically compressed air, are mainly used to dehumidify compressed air which is used in a great variety of pneumatic, industrial, etc. uses, which will not be further described as they are well known to the person skilled in the art.
To this purpose, said compressed air, usually supplied as a compressed air flow, is introduced inside said apparatus which comprises an inner path, and, after having been dehumidified, it is re-circulated outside at a substantially unvaried pressure.
Very synthetically, herein it is only reminded that the compressed air flow is caused to flow through a first branch of a heat exchanger, whose second branch is connected to a cooling means, of the known type, and which is its final cooling element, better known as “evaporator”.
While passing by said first branch, the compressed air flow is cooled up to a value lower than the dew point; as a direct and desired consequence the dampness contained in the air is condensed and therefore changed into water, generally formed by droplets, which collect on the walls or fall directly down.
Then, after having been separated from the air, because of gravity, said water is collected in the lower part by means of appropriate collecting means.
Finally said water is expelled outside the apparatus by means of suitable conduits and related control means, such as operated valves etc., known per se.
Various apparatuses apt of carrying out the dehumidification process of a compressed air flow have been devised and are currently actuated.
Common element and characteristic of such apparatuses consist in that not only one but two heat exchangers are used; the first heat exchanger, which does not contain said evaporator, is an exchanger of the gas/gas type.
A first branch is caused to be passed by the still warm and damp air flow before being introduced into said first exchanger; the relative second branch is caused to be passed by the air flow exiting the first heat exchanger.
The function of said first exchanger is double and known:
The first function is that of pre-cooling the compressed air flow before entering the first exchanger, with the clear purpose of achieving energy saving because the heat amount which has to be removed from the compressed air flow in order to reach the related dew point, is reduced in direct function (even if not proportional) of the lowering of the air temperature at the first exchanger inlet; therefore the less heat is removed from the compressed air the less will be the energy required by the cooling circuit;
The second function consists in heating the compressed air exiting the second heat exchanger; in fact this air, just dehumidified, is usually at a very low temperature, and often not usable just for that reason, in fact it can cause icing, external condensation in other parts, etc.
Notoriously, said heating function is, in fact, given to the air flow entering the heat exchanger upstream the second exchanger, because said exchange function is completely advantageous for both air flows, and it is obtained without any energy costs, and rather the air pre-cooling makes it possible a certain energy saving as explained.
From patent EP 1 464 887 A1 it is known an apparatus for the dehumidification of a compressed damp air flow; it comprises a first exchanger and a second exchanger arranged vertically one on the other, and wherein between the two exchangers it is arranged a compartment 38 which besides isolating the two exchangers one from the other, above all, it lets the air flow coming from the second exchanger 30, and along the second channel 22 pass between said two heat exchangers so as to be able to pass again through the first heat exchanger 24, cooling in this way the incoming damp air, as explained above.
Anyway, the fact that the second channel 22 is completely adjacent to second heat exchanger gives rise to an easily identifiable inconvenience; in fact, the air which passes by said channel 22 is generally warmer than the wall which is in common with the second heat exchanger and, therefore, it tends to be slightly cooled.
This is negative from the point of view of the dehumidifier efficiency as this second heat exchanger is consequently heated albeit of a little. As a final consequence it reduces its energy efficiency, but, above all, it is completely unsatisfying for the purpose of achieving a very high reduction of the residual dampness.
From patent EP 1 293 242 A2 it is known a type of heat exchanger for compressed air provided with a path of the conduits, which carry the air flow to be dehumidified, passing through both a first heat exchanger working as explained above and that is as a “pre-cooler”, and a second heat exchanger in thermal exchange relation, as usual, with a cooling circuit of the type known per se, usually an evaporator.
Patent EP 1 593 924 provides a type of dehumidifier with two heat exchangers substantially similar to the previous one, from which it differentiates only for a different architecture and geometry of the conduits of the various gases; it substantially has the same inconveniences as the previous one and, therefore, for brevity, its detailed discussion is avoided.
But the most serious inconvenience, which is common to all the above discussed patents, consists in that, even in the most favourable case that is when the conduits which form said two heat exchangers are arranged in counter-flow between them maximizing the thermal exchange efficiency, anyway, the residual dampness percentage cannot practically and conveniently be removed because a further reduction of the dampness would require a dehumidifier with rooms and higher costs in an exponential way, as it is well known to the skilled in the art.
To overcome such limit, patent EP 2 263 778 A1 shows how to realize an hybrid apparatus for the dehumidification of a compressed air flow and which comprises, besides a dehumidifier of the conventional type and similar to those of the patents discussed above, also a further dehumidification stage using hygroscopic and renewable means of absorption.
The solution herein described is certainly efficient and adequate in order to achieve a radical reduction of dampness, much higher than that which can be obtained with dehumidification apparatuses provided with heat exchangers only, as seen above.
Moreover, said solution is complicated and onerous from the constructive point of view as the two heat exchangers are practically separated and, therefore, little compact and so also with an inferior energetic efficiency; such limit is absolutely unacceptable in a highly competitive commodities sector as the one under discussion.